The ability to control a variety of functionalities with external stimuli is one of the
main issues in correlated oxides and their heterostructures. Due to the extreme
sensitivity of those material systems to external stimuli, the control of the versatile
functionalities can achieve unique phenomena, such as metal-insulator phase transition,
which can be applicable for various future electronic devices that require high
sensitivity and steepness. Among the various stimuli, (intrinsic or extrinsic) atomic
defects have a strong influence on the d-band filling, which is the core concept of
correlated electronic systems.

Hydrogen, the smallest and the lightest one among atomic elements, is reversibly
incorporated into the interstitial site of vanadium dioxide (VO2), a 3d1 correlated metal
oxide undergoing metal-insulator transition at ~ 68 °C, and then induces dramatic
electronic phase modulation. Here, we present that hydrogenation can be achieved up to
two hydrogen atoms per VO2 unit cell, and hydrogen is reversibly absorbed into and
released out of VO2 without destroying the lattice framework due to the low
temperature annealing process [1]. More importantly, this massive hydrogenation
process allows to elucidate phase modulation of vanadium oxyhydride (HxVO2),
remarkably demonstrating two-step insulator (VO2) – metal (HxVO2) – insulator (HVO2)
phase transition during inter-integer d band filling. Furthermore, this massive
hydrogenation was also universally observed in facet-dependent [2] and E-field-controlled [3] phase modulation using precisely controlled experiments. Our finding not
only shows the possibility of reversible and dynamic control of topotactic phase
transition in VO2, but it also opens up the potential application of functional defects in
correlated oxides for various electronic applications.